I've never had access to a try gun and someone gifted with stock fitting but as suggested by Tatume that is probably the very best way to answer your question. The next best thing is Daryl's advice which is right on mark. The try gun sorts out a person's individual body variances that he talks about from the manufactures' one-size-fits-all standards.

I have two shotguns that I shoot well. As suggested by Daryl when the gun is mounted with your eyes closed and you find good alignment without adjusting your position after opening your eyes that is what you are looking for. Both of these shotguns align well using this method and I have recorded the measurements as my ideals.

I recently completed a Mauser and a Ruger #1 project and I nearly duplicated these shotgun ideal measurements with both. The #1 has a length of pull of 14 1/8", 3/16" cast off and 1/8" toe in. I modified the drop at heel from the ideal to accommodate the higher sighting plane of the express sights relative to the shotguns' low solid ribs. Both of these stocks were rough machine inletted for me and I specifically requested that the butts be left very large. Cabinet patternmaker's rasps were used to do the rough shaping after the inletting was done and I could start testing and adjusting fit. These rasps remove material quickly and leave smooth surfaces that require little final work. The 14 1/16" length of pull on the Mauser allows the gun to remain fully mounted during recharging without getting poked in the eye with the bolt or having to adjust the head relative to the sight plane. The end result was two rifles that mount and align like my shotguns. My oversize wood effectively became the adjustable try gun as I removed wood to accomplish fit.